1. Technical Field
This invention relates to an accelerator for use with hydraulic jars in a drilling environment and, in particular, to a gas-filled accelerator for use with double acting hydraulic jars.
2. Description of the Related Art
Drilling jars have long been known in the field of well drilling equipment. A drilling jar is a tool employed when either drilling or production equipment has become stuck to such a degree that it cannot be readily dislodged from the wellbore. The drilling jar is normally placed in the drill string in the region of the stuck object and allows an operator at the surface to deliver a series of impact blows to the drill string via a manipulation of the drill string, such as by lowering and raising the drill string. Hopefully, these impact blows to the drill string are sufficient to dislodge the stuck object and permit continued operation.
Drilling jars contain a sliding joint which allows relative axial movement between an inner mandrel and an outer housing without allowing rotational movement therebetween. The mandrel typically has a hammer formed thereon, while the housing includes an anvil positioned adjacent the mandrel hammer. Thus, by sliding the hammer and anvil together at high velocity, they transmit a very substantial impact to the stuck drill string, which is often sufficient to jar the drill string free.
In some instances it is desirable to greatly enhance the force of the impact blows so that a much larger hammering force can be applied to a stuck object. Typically, the force of the drilling jar has been enhanced by adding an accelerator to the drill string. The accelerator is used to store energy until the jar is triggered. When the jar is triggered, the accelerator quickly releases its stored energy and accelerates the hammer of the drilling jar to a very high speed. The force of the impact is, of course, related to the square of the velocity, thus, the hammer force is greatly enhanced by the accelerator.
Drilling jars have been developed that are capable of delivering hammer blows in both an upward and downward direction. For example, U.S. Pat. No. 4,361,195, issued Nov. 30, 1982, to Robert W. Evans, describes such a double acting drilling jar. Double acting accelerators have also been developed, such as that described in U.S. Pat. No. 5,232,060 issued Aug. 3, 1993 to Robert W. Evans.